In the construction sector and auxiliary sectors, and also in the case of renovation measures, such a detection device, also referred to as a metal detector, is used, for example, to check whether or not a wall to be drilled through contains metallic objects. In general, a detection device may be used for locating electrically conductive objects, for example, also pipelines, in the floor or in a wall.
A device mentioned at the outset, which is also referred to hereafter as a detection device, may fundamentally be based on various measuring principles. One measuring principle is formed using a so-called continuous wave method, in which a continuous, typically low-frequency current is generated in a transmitting coil in the frequency range of several tens of kilohertz. With the aid of a receiving coil, a received signal, which arises due to an electromagnetic coupling between an electrically conductive object in the substrate and the emitted signal, is received and typically analyzed in the spectral range. In this measuring principle, either one measuring signal of a single frequency or a measuring signal which includes signal components in various frequency ranges is emitted. A detection device which is based on the above-described measuring principle is also referred to as a CW metal detector, CW standing for “continuous wave.”
A detection device may also be based on a so-called pulse induction measuring principle. A pulse generator typically generates a pulse-like electrical current, which is applied to a coil of a transmitter, also referred to as a transmitting coil. A pulsed magnetic field is thus generated, which impinges on a conductive object located in the substrate. According to electromagnetic interactions, the pulsed magnetic field induces an eddy current in the electrically conductive object, which decays after a certain time as a function of the transmitter current pulse pattern. The induced eddy current in the electrically conductive object then generates a secondary magnetic field, which is received by a receiver, e.g., a coil, in the detection device. This secondary magnetic field is typically received with the aid of a coil of the receiver referred to as a receiving coil, in which the secondary magnetic field induces a receive current, which is then amplified by an amplifier connected downstream from the receiving coil. The amplified received signal is conducted to a so-called gate, which represents a barrier with respect to time. The gate is operated by a previously known detection device in such a way that it is only open for a very specific period of time after the application of a current pulse to the transmitting coil. For example, a delay unit is connected between the pulse generator and the gate, which prompts an opening of the gate after a fixed time after the triggering of the transmitter current pulse. The function of the gate is thus to eliminate an influence of the directly received transmitting pulse and it allows the receiving coil to nonetheless detect the secondary magnetic field. The output signal of the gate is typically integrated by an integrator, which sums the cumulative signal over multiple pulses and makes this signal available for a display device. A detection device which is based on this measuring principle is also referred to as a PI metal detector, PI standing for “pulse induction.”
One or the other of the two above-described measuring principles is alternately implemented as a function of the field of use of the detection device and as a function of requirements which are placed on the detection device, for example, precision and/or power consumption. A metal detector, which may carry out both the CW and also the PI measuring principle, is known from the publication DE 699 17 030 T2. The metal detector described therein has a transmitter unit having a number of transmitting coils and a receiver unit having a number of receiving coils. In addition, the metal detector includes a so-called signal processing apparatus for correcting shifts in the case of measured components of received magnetic field gradients. For example, a received signal which has been distorted due to an incorrect orthogonal alignment of receiving coils in the receiver of the metal detector is thus post-corrected. However, this may be very time-consuming.